Relapsing fever Borrdia infections provide an excellent experimental system for comprehensively understanding the pathogenesis strategy of antigenic variation and the systems biology and evolutionof an infectiousdisease. The long term goals of the project are to (a) define the molecular basis for antigenic variation and pathogenesis by relapsing fever Borrelia spp., (b) characterize host responses that select for antigenic variantsand other evasion strategies by the parasite, (c) delineate the organismal and population biology of relapsing fever spirochetes in their mammalian reservoirs and arthropod vectors, (d) investigate the evolution of antigenic variation of relapsing fever Borrelia spp., and (e) integrate the findings from studies for goals (a) to (d) through mathematical modeling and computer simulation. Specific aim 1: Genetic basisfor antigenic variation. The first hypothesis for this aim is that antigen switches are the result of a form of homologous recombination that repairs a break at the expression site with an archived gene. To test this hypothesis recombination-deficient mutants of & hermsii will be produced. The second hypothesis is that the organization of the archival genes and non-coding repetitive sequences in part determines switch rates. We will investigate this by further characterizing the sequences of archival sites. The third hypothesis for this aim is that the diversity of the variable membrane protein genes is largely attributable to intragenic recombination within a single strain and not through the accumulation of point mutations or lateral gene transfer. We will test this by molecular evolution analyses of the expressed and archival variable genes in different strains of B. hermsii and in different species of Borrelia. Specific aim 2: Slrnclure-fanction studies of variable membrane proteins. For this aim further crystallographic studies of the Vsp proteins and their mutants wili be carried out and work on the structure of one or more Vlp proteins will begin. The hypothesis is that while the major structural features of the Vsp and V!p proteins are maintained across the respective repertoires, significant differences in antibody reactivities and host cell interactions by the proteins are determined by local structural polymorphisms. The general approach to this hypothesis is to further characterize functional differences between different Vsp and Vlp proteins under in vitro and in vivo conditions and to examine the effect of mutations on functions and antibody interactions of the proteins. Specific aim 3: Biology oj parasitism of mammalian hosts by relapsingfever spirochetes. The major hypothesis for this aim is that the parameters and dynamics of experimental relapsing fever will eventually be sufficiently elucidated at the molecularto organismal levels that an infection can be accurately simulated by computers. We will evaluate this by developing models that are based on currently available data about infection dynamics and then use these early models' deficiencies to direct us in further experimentation and modelbuilding.